1. Field of the Invention
The present invention relates to pressure-limiting valves, and, more particularly, to a ventilation check valve which is adapted for use with an internal combustion engine to control the suction effect on a pollutant ventilating line which is connected to the engine intake.
2. Description of the Prior Art
It has become common practice, in connection with present-day intensified efforts to abate automobile-created pollution, to install fumes ventilation lines which draw otherwise freely escaping pollutants from the engine crankcase and from the fuel tank vent into the intake system of the internal combustion engine, where the pollutants become admixed to the combustion mixture of fuel and air.
For the effectiveness of ventilation and pollution abatement it depends whether a ventilation line is connected to the filter housing or to the manifold intake duct downstream of the carburetor. Obviously, the suction action on a ventilation line connected to the filter housing is much less than on a ventilation line which is connected to the manifold intake duct. On the other hand, the negative pressure in the manifold intake duct undergoes much wider fluctuations, from a moderate negative pressure at open-throttle operation to an elevated negative pressure, when the carburetor throttle is in its near-closed idle position. A ventilation connection to the filter housing has the additional disadvantage that, when the engine is at standstill, the filter housing is open to the atmosphere, via the filter cartridge and the intake snorkel, thereby permitting pollutants to escape to the atmosphere.
It has already been suggested in connection with vacuum-assisted ventilation lines to counter the problem of the pressure fluctuations in the vacuum source by installing in the ventilation line a check valve which blocks the line when a certain level of negative pressure is reached. Such a device is disclosed in the German Gebrauchsmuster (Utility Model) No. 78 22 216. This known check valve features a flexible diaphragm spaced a short distance from an intake opening which serves as a valve mouth inside a valve housing with inlet and outlet connectors for continuing portions of the ventilation line. The outer side of the diaphragm is exposed to atmospheric pressure, in opposition to a compression spring which applies an opening bias to the valve.
At a certain level of negative pressure in the ventilation line, the difference between the pressures on the diaphragm from inside the valve housing and from the atmospheric pressure outside is sufficient to overcome the valve-opening spring bias, so that the diaphragm moves progressively closer to the valve mouth while compressing the spring, until it finally interrupts the ventilation flow by closing the check valve.
This prior art check valve, in order to allow for an adequate ventilation flow at a low level of suction, i.e. when the internal combustion engine operates at full load with the carburetor throttle in the open position, requires a comparatively large diameter of the valve mouth. The size of the valve mouth, in turn, determines the force with which the suction in the outlet duct of the valve holds the diaphragm against the valve mouth. It follows that, when the suction in the outlet duct is strong, the negative pressure in the valve housing has to drop considerably below the point at which the valve first closed, before it reopens. Only a stronger spring and a correspoindingly larger diaphragm will reduce this difference in the opening and closing responses of the valve.
The described prior art check valve is thus poorly suited for use in a pollutant ventilation line which is subject to wide swings in negative pressure at the suction source, as is the case in a line which is connected to the manifold duct of the engine. For, even if this type of valve is balanced for closing at a median level of negative pressure, it will still remain closed much of the time, except at high power output levels of the engine.